During the Cold War, shortwave radio broadcasts were critical to American espionage efforts.

"HM01 Spectogram," from Shadows of State. | Spectograms are the visual representation of sound, in this case the sound of a shortwave radio transmission. | (Lewis Bush)

Shortwave transmission sites — known as "number stations," because initial broadcasts were simply strings of numbers — were used by both the U.S. and Soviet governments to send propaganda to foreign countries, since these high-frequency transmissions could reach such great distances. But they were also a secure means of sending coded messages to intelligence officers operating in other countries. As long as the agent had the station, air time, and encryption code, he could receive a one-time message that only he could understand.

At the end of the Cold War, the number stations' transmissions decreased, but didn't disappear altogether. In fact, ghostly messages continue to reverberate across the airwaves.

"RAF Akrotiri, Cyprus," from Shadows of State. | An aerial view of a transmission site in Cyprus, composited out of satellite imagery. | (Lewis Bush)

Dial into an AM frequency on a shortwave radio and you might just happen across a code-laden radio broadcast. The stations can be silent for days and then spring to life, buzzing and beeping with Morse code, artificial human voices, songs, and even nursery rhymes — all in a number of languages.

The sources of these mysterious messages are unknown. They are transmitted from anonymous, unlicensed shortwave radio towers whose locations are not always known.

In 2015, British photographer Lewis Bush decided to try to find them."[I thought] I'd just do a couple of months work on it," Bush said in an interview. "And that grew and grew into the project I have now."

Tracking down the unlicensed, unverified towers is no easy feat. America's Federal Communications Commission doesn't have any information on them, and government agencies will reportedly deny their existence.

So, Bush adapted intelligence-gathering techniques himself. Using public domain sources, shortwave radio enthusiasts, declassified documents from archives of intelligence agencies, and history books, he was able to triangulate the probable locations of 30 transmitter sites.

Once he collected enough information on a transmission site, he used satellite maps for visual clues. "A transmitter site that is very highly secured, for example, with double lines of fencing, guard towers, and so on, tends to suggest it might be worth doing some more research on," he said.

Finally, he created high-resolution composite satellite image of the site as well as spectograms of the signals themselves.

"In both cases, the process involves screengrabbing a huge amount of material and then composting it together in Photoshop," he said. "The spectograms are particularly challenging because they are constantly changing, so you have to stick with them until the signal ends or risk ending up with an incomplete spectogram."

Spectograms are the visual representation of sound. They illustrate the range of frequencies — or vibrations — of a voice, music, or whatever the source. High frequencies, like shortwave radio, have more tightly packed and frequent sound waves, which gives the images a frenetic quality. If you think of the image like a graph, time would by the horizontal axis and the frequencies — or vibrations — of sound are the vertical axis.

The bird's eye perspective of the transmission sites puts the viewer in the role of Big Brother, spying on these hidden locales. While the spectograms — with their aggressive linear formations — give off a ghostly abstract beauty.

"A big part of the book is the doubt and uncertainty that surrounds a topic like this," he said. "You can never know absolutely, and that's part of what I find interesting."

"I hope [the viewer] will understand just how subjective and slippery photographs of all sorts are," he said.

"I hope these images are haunting — both haunting as a reminder of a period in history when we came very close to destroying our planet, and haunting as a reminder that this moment is not as consigned to the past as we might have thought."